“对某人不起作用”可以用以下几种英语表达：
1、 Have no effect on sb.
示例：This medicine has no effect on him.（这种药对他不起作用。）
2、 Not work on sb.
示例：The new teaching method doesn't seem to work on this group of students.（这种新的教学方法似乎对这组学生不起作用。）
3、 Fail to have an impact on sb.
示例：Our attempts to persuade him failed to have any impact.（我们试图说服他的努力没有起到任何作用。）
4、 Not take effect on sb.
示例：The painkiller didn't take effect on her.（止痛药对她没有起作用。）
5、 Be ineffective on sb.
示例：The advertising campaign was ineffective on the younger demographic.（这次广告活动对年轻群体没有效果。）